Despite biochemical and anatomical data which indicate that serotonin or a related indoleamine is a transmitter in the mammalial retina, our understanding of the function of the endogenous indoleamine is extremely limited. The role of this transmitter will be studied using pharmacological manipulation of the rabbit retina. Both an isolated superfused eyecup and an intact arterially perfused preparation will be employed to study the effects of specific serotonergic antagonists on the responses of physiologically identified ganglion cells. Results up to this point have shown that indoleamine antagonists reduce ON responses in both ON center and OFF center brisk (X and Y) ganglion cells. Several other aspects of ganglion cell function will now be addressed. 1. What is the role of the endogenous indoleamine in the elaboration of the receptive field properties of W-cells? Specifically are only ON-pathways affected and does the indoleamine make any contribution to trigger features. (In the past the role of a given transmitter has been elucidated only when these W-cells were studied). 2. What is the role of the endogenous indoleamine in the photopic versus scoptic retina? The indoleamine-accumulating neuron is extensively connected to the rod bipolar cells, thus it is possible that the endogenous indoleamine has a specific role in scotopic retina. 3. What role does the endogenous indoleamine have in the modulation of ganglion cell responsivity and sensitivity? The hypothesis which has emerged from previous work suggests that this transmitter serves to increase the sensitivity to increments of light. This hypothesis will be tested by studying the effects of serotonergic drugs on the responsivity and sensitivity of ganglion cells. The rabbit retina has proven to be a useful model system in which to develop generalizations about transmitter function which have proven valid in other retinae and also other regions of the CNS. Thus, elucidating the role of the endogenous indoleamine in rabbit retina will contribute to our understanding of visual processing and to the understanding of the function of serotonin in the mammalian CNS.